PlatformFontMac: Use a default NSFont when an invalid font is passed.

ui/gfx/font_fallback_mac.mm

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/font_fallback.h"
 
 #include <CoreText/CoreText.h>
 #import <Foundation/Foundation.h>
 
 #include "base/mac/foundation_util.h"
@@ skipping 22 matching lines @@
   base::ScopedCFTypeRef<CTFontDescriptorRef> lhs_descriptor(
       CTFontCopyFontDescriptor(lhs));
   base::ScopedCFTypeRef<CTFontDescriptorRef> rhs_descriptor(
       CTFontCopyFontDescriptor(rhs));
   return lhs_descriptor.get() == rhs_descriptor.get();
 }
 
 }  // namespace
 
 std::vector<Font> GetFallbackFonts(const Font& font) {
+  DCHECK(font.GetNativeFont());
   // On Mac "There is a system default cascade list (which is polymorphic, based
   // on the user's language setting and current font)" - CoreText Programming
   // Guide.
   NSArray* languages = [[NSUserDefaults standardUserDefaults]
       stringArrayForKey:@"AppleLanguages"];
   CFArrayRef languages_cf = base::mac::NSToCFCast(languages);
   base::ScopedCFTypeRef<CFArrayRef> cascade_list(
       CTFontCopyDefaultCascadeListForLanguages(
           static_cast<CTFontRef>(font.GetNativeFont()), languages_cf));
 
   std::vector<Font> fallback_fonts;
+  if (!cascade_list)
+    return fallback_fonts;  // This should only happen for an invalid |font|.
 
   const CFIndex fallback_count = CFArrayGetCount(cascade_list);

[tapted, 2017/06/28 03:46:10]

Currently it's this line that crashes -- null can't be passed to CFArrayGetCount

   for (CFIndex i = 0; i < fallback_count; ++i) {
     CTFontDescriptorRef descriptor =
         base::mac::CFCastStrict<CTFontDescriptorRef>(
             CFArrayGetValueAtIndex(cascade_list, i));
     base::ScopedCFTypeRef<CTFontRef> font(
         CTFontCreateWithFontDescriptor(descriptor, 0.0, nullptr));
     if (font.get())
       fallback_fonts.push_back(Font(static_cast<NSFont*>(font.get())));
   }
 
@@ skipping 14 matching lines @@
   base::ScopedCFTypeRef<CTFontRef> ct_result(
       CTFontCreateForString(ct_font, cf_string, {0, text_length}));
   if (FontsEqual(ct_font, ct_result))
     return false;
 
   *result = Font(base::mac::CFToNSCast(ct_result.get()));
   return true;
 }
 
 }  // namespace gfx